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Delboeuf illusion. An illusion created by the Belgian philosopher Franz Joseph Delboeuf. The two red circles on the left have the same diameter, as do the two red circles on the right. However, they appear to differ in diameter within the two pairings. This is a result of the blue circles that are connected to each of them. An effect known as assimilation causes the brain to reduce the perceived distance between neighbouring objects, and hence view the red circles as larger or smaller according to the placement of the blue circles.
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